Electrically conductive floor covering for use in explosive hazard areas



Feb. 18, 1964 M. 1'. ABEGG ETAL ELECTRICALLY CONDUCTIVE FLOOR COVERING FOR USE IN EXPLOSIVE HAZARD AREAS Filed 001:. 14, 1959 INVENTORS. M. TAYLOR ABEGG OWEN D. BmTLER BY JACK D. DELIGANS FORREST s. NICHOLSON M Q,

'IIIIIIIJ'J ATTORNEY.

United States Patent This, invention relates to building materials and, more particularly, to a floor surfacing characterized'by a uniform high resistance electrical'conductivity to ground for use in explosive hazard areas.

Any building where'explosive materials are handledor stored is required by general considerations of safety to be provided with a flooring having a high resistance electrical conductivity to ground. For example, the Army Ordnance SafetyManual specifies that the resistance to ground, as measured with a megger between a=two and one half inch diameter steel slug placed'at any point on the floor surface and electrical ground, must be within the range of 25,000 ohms to 250,000 ohms. This limited conductive property of of the floor surfacing material is required inorder to prevent. the accumulation of any static electricalcharge which could in turn possibly' generate a-spark andcause an explosion. p

In addition to the requirements for electrical conductivity of the .floor surfacing materials, safety also requires that the floor surface be free of all .cracks or crevices or other cavities where explosive materials might lodge and thus create a dangerous condition. Of course, surface material ;o f ahard metallic nature would be undesirable since such material wouldproduce sparks if struck by another metal. object. Furthermore,.the high conductivity of a metal surface is unsafe where the use of electrical tools and appliances in the building is contemplated. 1

The present invention considered in its broad aspects provides a. composite floor surfacing material comprised of a powderedelectrically conductive material suspended in a plastic material disposed as a uniform layer over a conductive sheet or netting. The conductive sheet or netting is in turn installed over a sub-base and electrically grounded. The invention also provides methods for the convenient installation of the described conductive floor surfacing material. I

It is therefore an object of this invention to provide a floor surfacing material and method of manufacture thereof for use in explosive hazard area's.

Another object of the invention is to provide. a floor surfacev having a uniform specified electrical resistance to ground. If

Still another object of the invention is to provide an electrically conductive floor surfacewhich satisfies specified safety requirements.

A further object of the invention is to create a floor surface free of all cracks, crevices, and other cavities suitable -for use iiifexplosive hazard areas.

Still another object of this invention is the provision of a conductive j floor surfacing material whichv will not generate a .spa rk by collision, with a metallic object.

A still further object of the present invention is to produceeconomically a conductive floor surface of particular electrical properties that has a good appearance and good wearing qualities,

Other objects and advantages of the invention'will becomeapparent by reference to the following specification taken in conjunction withv the accompanying drawing, in which: I

FIGURE 1 is a fragmentary perspective view of the Patented Feb. 18,

2 floor covering ofthe invention as typically installed; and

FIGURE 2 is anenlarged cross sectional view of a typical portion of the floor of FIGURE 1, taken along plane 2- 2 thereof. f f

Referring to FIGURE 1, thereis'shownaroom'12 having a fioor covering l lconstructed in accordance with the invention, such floor rests on a smooth subfloor 13 of conventional nature such as a concrete slab. Such covering 11 includes a highly conductive layer, generally provided as. a continuous sheet or a netting 14 of, an electrical conductor with metals, e.g., copper and aluminum being particularly suitable. Sheet 14 ordinarily must 'lie in uniform even contact with .subfloor 13 to provide uniform thickness and resistivity of the covering. The meshsize of screen if used is not critical but shouldbe preferably less thanone inch. In the eventa metal sheet is employed, the sheet is cemented or otherwise secured to the subfloorso that the completed covering will not raise or spring away from the smooth subfloor 13 in any substantial area. y y 1 A ground wire 16, preferably of copper or other low resistance electrical conductor connects sheet 14 with a proper external ground .17 in accordance with the practices of the electrical industry. In order to insure good conductance of the various wires ofthe netting 14 to the ground wire 16, it is desirable to solder or otherwise join the. mesh wires together so in any event a continuously conductive layer 14 is provided.

To complete the main body of the floor covering 11 a layer 18 of a composite conductive thermosetting plastic composition is applied over conductive layer, by alternative techniques. In the event that layer 14 comprises a continuous sheet, compositionlayer 1'8 is'tightly adherent thereto; however, if conductive screenis employed, as shown in the drawing, the composition permeates the interstices thereof. and is adherent to subfioor 13, with the screen being embedded therein. I, I i

The aforesaid composite composition consists essentially of a plastic or resin binder capable of setting by chemical action and a particulate conductive material. For present purposes the conductive material is preferably a powdered metal and particulate aluminum metal powder is the material of choice. The binder and par-ticulate metal are mixed in such proportions that the bulk resistivity falls within the desired range for specified thicknesses of the cured composition. The peculiar properties of aluminum metal particles which bear a thin oxide coating in combination with'the plastic resin binder is believed to account in large part for the most satisfactory properties which the present floor covering exhibit for the indicated purposes.

Viscous epoxy resins have been found most satisfactory for use as the binder. Such resins are generally produced by partial condensation of epichlorhydrin and a bi-funcether and other materials may be added for copolymerization and plasticizers such as Thiokol liquids serve as plasticizers. More specifically, such epoxy resins are cured by adding hardeners and a catalyst with curing being effected at room temperature or at slightly elevated temperatures dependent upon catalyst used and variations in formulation. Many variations and modifications of suitable and equivalent epoxy resin formulations are availdepend upon utilizing long chain epoxy substituents in the resin. Other resins such as polyester and phenol-aldehydes which are processed similarly to the epoxies can also be employed if less chemical resistance, adhesiveness, durability and the like are tolerable.

Comminuted aluminum metal of the standard commercial grades is generally employed in producing the compositions. Other comminuted powdered or finely granulated aluminum powders may also be employed while similar forms of other metals may be utilized provided proportions are adjusted to provide appropriate re- .sistivity. Ordinarily, about 1 to 2.50 parts of comminuted aluminum to 1 part of epoxy resin admixed with hardener and catalyst and V100 part Thiokol S yields an easily processed viscous plastic mix which, in thicknesses of about A inch, yields upon curing a resistivity in the range under the conditions specified in the Army Ordnance Manual discussed above.

Other details of the invention will be apparent from the following specific examples illustrating methods of creating the conductive floor covering. As discussed above, a relatively smooth wooden, concrete or other floor is covered with an adherent continuous grounded metallic sheet or by a continuous grounded metal screen, the latter construction being considered hereinafter for illustrative purposes, g

In accordance with one method a viscous composite plastic mixture of the character described is blended as in a stirred vessel and is poured onto the screen covered surface (or metal sheet covered surface) and is spread to the desired thickness, e.g., inch by conventional techniques and the material is allowed to cure with infra red or other heating if faster and more complete cures are desired. A specific composition corresponding to the following formula was so employed:

Shell epoxy resin 828 100 parts by weight. Thiokol S 50 phr. Agent D 15 phr. Aluminum powder 200 phr.

1 Parts per hundred of resin.

Subsequent to curing the surface waslightly sanded to provide a smooth finish and adequately expose aluminum granules and powder particles.

Alternatively, inch thick tiles 2 ft. square were cast of the indicated plastic mix for convenient handling; however, .tiles of other convenient sizes or shapes may likewise be employed. To install such tiles a coating of the uncured plastic mix used as a cement 19 is placed over the subfloor 13 to cover the mesh of the screen using sufficient to just cover the mesh and fill the spaces between the wires thercof. The tile is then placed over the cement while still in the plastic state and the tile pressed down firmly in place in substantially the same manner as is used in laying any tile floor. However, in laying this conductive tile, a gap 20, see FIGURE 2, is provided between the adjacent tiles. This gap may be to inch for example. After the tiles have been laid and securely pressed into place uncured plastic cement mix 19 is disposed in the gaps 20 between tiles, making certain that all the gaps are entirely filled. The tiles 18 may then be cured with heat and optionally be covered with a very thin coating of the cement 19 to provide a smoother surface. The cement 19 is permitted to harden and after this hardening process is completed, the fioor surface is sanded so that a smooth even finish is obtained on all portions of the'surface.

In addition to meeting the preferred safety requirements, the floor surfaces provided under either of these embodiments will have other desirable qualities such as pleasing appearance and good wearing characteristics. The texture is also very good and when made slightly rubbery as indicated tends to reduce the noise level.

It will be evident to those skilled in the art that many other variations and modifications are possible within the scope and spirit of the invention, and it is thereforenot. intended to limit the scope of the invention except as defined by the following claims.

What is claimed is:

1. An electrically conductive floor covering structure for use in explosive hazard areas comprising a hard substantially smooth subfloor area, a continuous sheetof a low resistivity electrically conductive metallic material provided with an exterior ground connection and bonded uniformly with one surface in contact with said subfloor area, and a uniform thickness layer of a composite electrically conductive composition bonded to the second surface of said sheet of electrically conductive material, said composition comprising a cured admixture of an epoxy resin, a curing agent for said epoxy resin, and comminuted aluminum metal having the normal oxide coating thereon in amounts providing an electrical resistance in the range of 25,000 to 250,000 ohms as measured between a two and one half inch diameter steel plug in contact with the surface thereof and said sheet of metallic material.

2. An electrically conductive floor covering structure second surface of said sheet of electrically conductive:

material, said composition comprising a cured admixture of an epoxy resin, a curing agent for said epoxy resin, a fluid Thioplast elastomer in an amount of up to 70% by weight of said resin, and comminuted aluminum metal having the normal oxide coating thereon in proportions of about 1 to 2.50 parts of metal to 1 part by weight of saidepoxy resin providing an electrical resistance in the range of 25,000 to 250,000 ohms as measured between a two and one half inch diameter steel plug in contact with the surface thereof and said sheet of metallic material.

3. A floor covering structure as defined in claim. 2 wherein said Thioplast is present in an amount of about 50 parts per hundred of resin by weight and said aluminum powder is present in an amount of about 200 parts per hundred by weight of resin.

4. A floor covering structure as defined in claim 2 wherein said sheet material is a metallic screen having a mesh size less than about one inch and said Thioplast is present in an amount of about 50 parts per hundred of resin by weight and said aluminum powder is present in an amount of about 200 parts per hundred by weight of resin.

5. An electrically conductive floor covering'tile' for use in constructing a floor structure in an explosive hazard. area comprising a composite admixture of anlepoxy thermosetting resin binder, a curing agent for said resin,

and comminuted aluminum metal having the normal" oxide coating adherent thereonyinv proportions "of '1 t6 2.50 parts of metal to 1 part by-wei'ght, of said epoxy resin, said composition being "-rnolde'djand cured to'pr'o and a sheet of electrical 'c'bntlut:tor in contact with .the'

second side thereof. l 6. A floor covering tile as defined in claim ,5 wherein 5 6 a fluid Thioplast is included in said composite admixture 2,456,373 Campaigne Dec. 14, 1948 in proportions in the range of 1 part to 70 parts by 2,457,299 Biemesderfer Dec. 28, 1948 weight of said resin, 2,718,829 Seymour et a1. Sept. 27, 1955 I 2,729,770 Robbins Jan. 3, 1956 References Cited in the file of this patent 5 2,851,639 Ford Sept, 9, 1953 U T STATES PATENTS 2,956,039 Novak et a1. Oct. 11, 1960 2,287,766 Davis June 30, 1942 T ER R FEREN ES 2,325,414 Mcchesl'ley y 27,1943 Publication by Rubber and Asbestos Corporation, 2,351,022 Donelson June 13, 1944 10 Jerome L. Been, leaflet #145, March 1956 (reprinted 2,378,623 Donelson June 19, 1945 from Modern Plastics, March 1956). 

1. AN ELECTRICALLY CONDUCTIVE FLOOR COVERING STRUCTURE FOR USE IN EXPLOSIVE HAZARD AREAS COMPRISING A HARD SUBSTANTIALLY SMOOTH SUBFLOOR AREA, A CONTINUOUS SHEET OF A LOW RESISTIVITY ELECTRICALLY CONDUCTIVE METALLIC MATERIAL PROVIDED WITH AN EXTERIOR GROUND CONNECTION AND BONDED UNIFORMLY WITH ONE SURFACE IN CONTACT WITH SAID SUBFLOOR AREA, AND A UNIFORM THICKNESS LAYER OF A COMPOSITE ELECTRICALLY CONDUCTIVE COMPOSITION BONDED TO THE SECOND SURFACE OF SAID SHEET OF ELECTRICALLY CONDUCTIVE MATERIAL, SAID COMPOSITION COMPRISING A CURED ADMIXTURE OF AN EPOXY RESIN, A CURING AGENT FOR SAID EPOXY RESIN, AND COMMINUTED ALUMINUM METAL HAVING THE NORMAL OXIDE COATING THEREON IN AMOUNTS PROVIDING AN ELECTRICAL RESISTANCE IN THE RANGE OF 25,000 TO 250,000 OHMS AS MEASURED BETWEEN A TWO AND ONE HALF INCH DIAMETER STEEL PLUG IN CONTACT WITH THE SURFACE THEREOF AND SAID SHEET OF METALLIC MATERIAL. 